This invention relates to optical fibers, and, more particularly, to the proof testing of optical fibers to ensure minimum strength levels.
Optical fibers consist of strands of optically pure glass fiber processed so that light beams transmitted therethrough are subject to total internal reflection. A significant fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds or thousands of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types, as compared to electrical signals carried on metallic wires. Moreover, the optical fibers are light in weight and made in large part from a highly plentiful substance, silicon dioxide.
Optical fibers are typically fabricated by preparing a preform of glasses of at least two different optical indices of refraction, one inside the other, or a single glass composition with a coating that ensures total internal reflection, and processing the preform to a glass fiber by drawing, extruding, or other method. The glass fiber is coated with a polymer layer termed a buffer coating to protect the glass from scratching or other damage. The optical fiber therefore includes two components, the glass fiber and the overlying buffer coating. As an example of the dimensions, in a typical configuration the diameter of the glass fiber is about 0.005 inches, and the total diameter of the optical fiber, including the glass fiber and the polymer buffer coating, is about 0.006- 0.010 inches.
In some applications, the optical fiber must have sufficient strength to bear some loading in use, in addition to transmitting light. In one example, optical fibers are wound under tension onto a bobbin and packaged in a canister, from which they are dispensed or paid out during use. The optical fiber must not fail during the winding and dispensing operations.
Glass fibers of the fine sizes mentioned above may be made with sufficient strength to carry the required loadings during winding and dispensing operations, and a great deal of manufacturing technology has been developed to fabricate glass fibers having such strengths. (The buffer coating itself has little strength, and is present to prevent introduction of surface defects into the glass fibers.) However, the strength of glass fibers is notoriously sensitive to even the slightest imperfections that may be present in the glass fibers. One slight imperfection in the glass fiber portion of over a thousand meters length of optical fiber may render the entire length of optical fiber useless. The buffer coating over the glass fiber is intended to prevent the introductions of imperfections into the glass fiber, but it is possible that such imperfections will be introduced during manufacture, transport to a canister winding facility, or storage before payout.
Proof testing is used to minimize the possibility of failures of the optical fiber during use, by inducing failure during testing, thereby permitting the discovery and repair of weak and flawed portions of the optical fiber. In conventional proof testing as presently performed, the optical fiber is loaded in tension to a preselected proof test load. If the optical fiber fails at any point along the loaded length, the break can be repaired by known splicing techniques that remove the region of the fiber containing the defect and reform the optical fiber without a defect present. If it is properly performed, proof testing works well for the testing of a material such as glass.
A conventional proof tester for optical fibers includes a pair of spaced apart rollers over which the optical fiber is fed, and a tensioning mechanism that applies a selected tensile force into the glass fiber portion of the optical fiber in the region between the rollers. The tensile force is selected to be greater than that which the optical fiber would experience during winding and dispensing operations, or such other use of the optical fiber as may be intended. The entire length of optical fiber, sometimes well over a thousand meters, is gradually passed over the rollers and through the tensioning region. If there is an imperfection in the glass fiber sufficiently large to cause failure under this proof test loading, the failure will occur during the testing and not during subsequent use. The weak region may then be repaired and retested to be certain that the entire optical fiber has the required mechanical properties.
Tensile proof testing is widely used for screening optical fibers to assure a minimum tensile strength. However, as the required strength level of the optical fibers has been gradually increased with increasing demands of use, it has been observed that the above-described tensile proof test procedure can itself cause damage to the buffer coating during testing as the optical fiber passes over the rollers, and that the damage to the buffer coating may later result in the introduction of post-testing imperfections in the glass fiber. Also, because the rollers tend to have diameters large enough to make the bending stress small compared to the tensile stress, the distance over which any particular length of the fiber is exposed to the proof stress is typically on the order of several feet. This long length results in the imposition of the stress on any particular length of fiber for at least a second. For many applications, shorter durations of stress are more advantageous, because significant new defects can be introduced if the period of stressing is too long.
There is therefore a need for a better approach to proof testing optical fibers, which achieves the required loading of the glass fibers and does not induce imperfections by virtue of the testing procedure. The present invention fulfills this need, and further provides related advantages.